For the purpose of establishing plural separate optical communications, optical fiber ribbons each of which has a plurality of optical fibers bundled in a tape-like shape have been available for these years. A plurality of such optical fiber ribbons are further bundled and the whole of them is covered with a sheath, thereby a single optical fiber cable is structured and then laid down. Such an optical fiber cable after being laid may be in some cases subject to a work referred to as “intermediate post-branching” in which one or more cores are drawn out and linked with another optical fiber cable. In carrying out the intermediate post-branching, first it is required that the sheath of the ribbons is split for the purpose of drawing out one or more cores and then each optical fiber ribbon is made to branch. This work usually requires specialized tools.
At a time of implementation of the intermediate post-branching, the sheath is split and thus a bend is given to the optical fiber, thereby transmission loss sometimes increases. In a case where the subject optical fiber ribbon includes an optical fiber just being used for communication, namely a “hot line” optical fiber, it is very important to prevent increase in transmission loss induced by bending.
Related arts are disclosed in Japanese Patent Applications Laid-open No. H09-80297, 2005-62427, 2005-292518, H01-138516, 2005-352510 and 2006-030684, and Utility Model Application Laid-open No. S59-22404.